This invention generally relates to an apparatus and method for removal of materials using laser light. More particularly, it relates to matching the wavelength of an ultrashort laser pulse to the absorption characteristics of the material being removed, in order to enhance the removal or ablation of one material, while leaving adjacent materials with different absorption properties undisturbed. Even more particularly, it relates to the use of ultrashort duration laser pulses to repair opaque defects on reflective photomasks. More particularly it relates to the removal of defects on reflective photomasks by matching the wavelengths of the ablative laser light to the peak of the reflectance of the underlying photomask substrate.
Photomasks are extensively used in the fabrication of integrated circuits on semiconductor wafers. While standard photomasks include a patterned absorbing or opaque film on a transparent substrate, a reflective mask includes a reflective substrate coated with an absorbing, opaque, or lower reflectivity patterned material. A metal, such as chromium, having a thickness on the order of about 1000 Å, is often used as the opaque or absorbing film. Other examples of absorbing or non-reflective materials include TaN, TaSiN and TaBN.
Fabrication of reflective photomasks first involves production of a reflective substrate. In the visible range of wavelengths, the substrate may be a piece of glass or other stable material. The substrate is then coated with an alternating series of dielectric films to form a reflective stack. Materials, whose optical thickness corresponds to one-quarter of the wavelength of the incident light, are deposited in layers onto the substrate to form a multilayer coating. The layered materials have alternating relatively high and low dielectric constants such that the reflected wavefronts from each interface constructively interfere in the backward or reflected direction. Reflectivities can approach 100% in the deep ultraviolet (UV), UV, visible, and near infrared (IR) regions of the optical spectrum. In some cases, the reflective substrate may be a metal which is highly reflective at a particular wavelength of light.
This construct may work effectively in a vacuum and in extreme ultraviolet (“EUV”) regions of the optical spectrum as well. In those cases, multilayer films can be made with alternating layers of absorbing and non-absorbing films, where thin layers of absorbing films are positioned at the node of the standing wave field within the multilayer stack. A phase shift of 180 degrees on reflection from each film produces constructive interference in the backward or reflecting direction. Reflectivities can approach 70-80% in the EUV region of the optical spectrum.
Upon production of the reflective substrate, an absorbing or non-reflective material, as mentioned above, is then deposited onto the substrate, followed by an electron beam or photon beam sensitive organic resist. The resist is exposed with a high resolution technique, such as an electron beam, and developed to form the desired pattern in the resist. This pattern is then transferred into the absorber by etching, leaving both opaque/non-reflective and reflective regions on the mask.
The above-described conventional photomask manufacturing process usually results in at least some imperfections, and defects are therefore frequently encountered during inspection of the photomasks. In advanced mask production, the defect rate per mask approaches 100%, which is unacceptable for cost-effective manufacturing. Defects are categorized as either “clear defects” or “opaque defects”. Clear defects are regions designed to have the absorber present, but which actually do not have absorber. Opaque defects are regions designed to be clear of absorber, but which actually do have absorber. FIG. 1 illustrates typical defects found on photomasks, such as a bridge defect, a bump or extension defect, or an isolated defect.
When a defect is a bridge defect connected to an adjacent absorber line, as in FIG. 1, conventional laser ablation may damage that adjacent line, undesirably removing some wanted absorber from the line. In addition, because a relatively high amount of thermal energy can be transmitted with the laser beam, the laser ablation step not only melts and vaporizes the unwanted metal defect region, it may also damage and remove a layer of substrate underlying and adjacent the opaque defect, producing roughness in the substrate. This damaged region of the quartz or glass substrate is also responsible for reduced reflectivity and altered phase of reflected light.
As an alternative to laser ablation, conventional focused ion beam (FIB) techniques offer a very controlled process for sputtering a small region of unwanted material. The ion beam can, in principle, be focused to a much smaller size than the laser beam. In addition, the ion beam physically sputters material, transmitting very little thermal energy to the mask. However there are a number of problems that limit the use of FIB for mask repair.
If the substrate is insulating, the ion beam rapidly charges the surface, and both the ability to aim subsequent ions and to use the ion beam to image the results is degraded. Second, while an opaque defect is being removed, substrate at the edge of the defect is attacked at the same rate, and the result is a “river bed” or trench of damaged substrate around the defect. The substrate in this region has altered reflectance and phase. Third, the focused ion beam species is typically gallium, and gallium has been found implanted into the substrate when the opaque defect is removed, causing reflectance losses. Fourth, the sputtering of material by the ion beam leads to ejection of material in all directions, and some of this ejected material may come to rest on adjacent edges.
A more general problem involves the ablation of a specific material, patterned or otherwise, that resides in a matrix of other materials, without damaging the desirable materials surrounding the specific material to be removed. What is needed then is an ablation process and apparatus or device that could be tuned to specific absorption properties of materials, thereby permitting the removal of one material, while leaving other materials in the matrix undisturbed.